1. Field of the Invention
This invention relates generally to a fuel cell system that employs a water vapor transfer (WVT) unit for controlling the humidity of cathode inlet air to a fuel cell stack and, more particularly, to a fuel cell system that employs a WVT unit for controlling the humidity of cathode inlet air to a fuel cell stack and a charge air cooler for controlling the temperature of the cathode inlet air.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, fuel cell membranes operate with a certain relative humidity (RH) so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity of the cathode outlet gas from the fuel cell stack is typically controlled to control the relative humidity of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the membrane is absorbed by the membrane and transferred to the cathode air stream at the other side of the membrane.
If the cathode exhaust gas has a relative humidity below 100%, then the water transfer efficiency of the WVT unit significantly decreases. To overcome this loss of water transfer efficiency, it would typically be necessary to increase the size of the WVT unit to achieve the desired stack inlet humidity. However, as the size of the WVT unit increases, the cost of the WVT unit significantly increases. Also, larger WVT units could present packaging problems in the fuel cell system.
The cathode inlet air is heated by the compressor. It is known in the art that the cathode inlet air can be cooled prior to being sent to the WVT unit so that the cathode inlet air is at the proper temperature for absorbing more water and water vapor. In one known system, the stack cooling fluid that is used to cool the fuel cell stack is also used to cool the cathode inlet air so that the temperature of the cathode inlet air is about the same as the stack temperature.